Surface imaging exoelectron microscope

ABSTRACT

An electron microscope employing the principles of exoelectron emission and induced photoemission is described. Surface variations which are microscopically and optically undetectable can be visually studied and/or photographed using the inventive microscope. Exoelectron emission from the surface of the substance is depended upon, and varies with, the characteristics of the surface even though the variations are microscopically small. The detectable characteristic variations include faults, cracks, homogeneity variations, surface variations, composition impurities, and other such surface characteristics. The exoelectrons emanate from such a surface in a pattern which includes the variations. The electron pattern is magnified to produce a visual image of the small variations.

.1 tes atem 1 H 9 9 Braunlich Sept. 11, 1973 SURFACE IMAGING EXOELECTRON Primary ExaminerJames W. Lawrence MICROSCOPE Assistant Examiner-T. N. Grigsby [75] Inventor: Peter F. Braunlich, Bloomfield Hills, AtmmeyLestar Hauacher et a]? Mich.

[73] Assignee: The Bendix Corporation, Southfield, [57] ABSTRACT Mich. An electron microscope employing the principles of exoelectron emission and induced photoemission is de- [22] Flled' July 1971 scribed. Surface variations which are microscopically [21] A pl. No.: 167,313 and optically undetectable can be visually studied and [or photographed using the inventive microscope. Ex-

52 US. Cl 250/307 250/495 B 250/213 VT lemon emissmfmmlhe l (the l l is [51] Int CL H0 31/56 37/26 H0 39/12 depended upon, and varies with the characteristics of [58] Field of Search 250/4975 A 49.5 B the Surface l 9 l f l" 2507213 cally small. The detectable characterist c variations include faults, cracks, homogeneity variations, surface [56] References Cied variations, composition impurities, and other such surface characteristics. The exoelectrons emanate from UNITED STATES PATENTS such a surface in a pattern which includes the varia- 3,5Q5 ,52l 4/1970 Wegmann et al. 250/495 B [i n The electron pattern is magnified to produce 3 2,894,160 7/1959 Sheldon 250/495 A visual image f the small variations 3,603,832 9/1971 Manley 250/213 VT' 24 Claims, 2 Drawing Figures PATENTEUSEPI H973 FIG. I PRIOR ART FIG.2

INVENTOR PETER F. BRAUNLICH ATTORNEY SURFACE IMAGING EXOELECTRON MICROSCOPE BACKGROUND OF THE INVENTION An understanding of the rudiments of existing electron microscopes and the phenomena of exoelectron emission is an aid to an understanding of the invention.

Exoelectrons are emitted from surfaces of many dielectric compounds and metals when heated or optically stimulated with light after having been exposed to various forms of radiation. Exoelectron emission also occurs when the substance is distorted by the application of forces such as bending, twisting or hammering. The emission occurs while distortion is taking place and continues thereafter, although it decays with time. The phenomena of thermally stimulated exoelectron emission is different from thermionic emission because it occurs at a much lower temperature. The phenomena of optically stimulated emission is different from photoemission, inasmuch as the wavelength of the stimulating light required for optically stimulated emission is longer than the wavelength required for photoemission. It is believed that exoelectron emission occurs because the exposing of the substance to radiation such as X ray, alpha or beta particles, gamma rays, or other types of radiation causes electrons within the material to be raised to energy levels above the equilibrium Fermi level in which they are trapped. Upon exposing the substance to temperatures which are in excess of the temperature at which the radiation exposure occurred but which are less than'the higher temperatures required for thermionic emission, the trapped electrons in a thin surface layer escape across the potential barrier at the The Sample or Pattern 10 to be viewed is placed in the microscope and illuminated with the radiation from a Source 12. Electrons are emitted from the surface of the sample when photons from Illuminating Source 12 impinge with the electrons. In FIG. 1 the illumination is provided by an Ultraviolet Light Source 12 which is used to illuminate Sample 19. Ultraviolet Source 12 is an intense source employing a Quartz Lens 13 to focus the energy onto the Sample 10. Photoemission occurs because the electrons within Sample 10 are given sufficient momentum by absorption of the photons from Source 13 to escape from levels below the Fermi level in the surface of the sample.

' A high potential electric field is established between the surface of Sample 10 and an Anode 14 so that the electrons leaving the surface of Sample 10 are accelerated toward Anode 14. The electrons are focused to an Aperture 16 within Anode 14 so that they pass to an Objective Lens 17 which serves as the first focusing lens of a three-stage magnetic electron microscope. An Intermediate Lens 18 and a Projector Lens 19 serve as the other two lenses of the microscope. After focusing by the Projector Lens 19, the electrons are directed to a screen or Photoplate 21 where they are used to produce a visual or photographic image. It will be noted that the image created by each of the lenses is reversed from the image which is input to that lens. This is indicated by the alternate orientation of arrows 22, 23, and

surface of the material into the gas atmosphere or the vacuum adjacent the substance surface.

Studies of exoelectron emission of various substances indicate that peak emission occurs at specific temperatures for specific substances. For example, maximum electron emission for lithium fluoride occurs at a temperature of approximately 150 centigrade. Accordingly, any study involving exoelectron emission of such a substance would be conducted by elevating the sample to 150 centigrade. i

Because exoelectron emission is dependent upon the surface of the sample under study, it is possible to visually display the surface characteristics of the substance or to photograph the surface characteristics of the substance by properly detecting the exoelectron emission. This is so because the number of exoelectrons'emitted from a particular point on the substance is dependent upon the characteristics of the substance at that particular point. As a consequence, the exoelectron emission across the surface of the substance varies in accordance with the variations of the surface characteristics of the substance. For this reason, cracks, pits, surface faults, and density variations in the homogeneity of the substance and other surface variations can be visually detected and displayed by detecting the exoelectron emission.

After the surface characteristics are detected, as evidenced by the exoelectron emission, a particular area of the surface or the entire surface can be magnified and visually viewed by the use of an electron microscope which greatly magnifies the detected image.

A schematic showing of a known type of electron microscope is shown in FIG. 1. The microscope shown is a known type of a photoemission electron microscope.

24which represent the images at the various stages within the microscope.

Photoemission electron microscopes of the type described with respect to FIG. 1 have many usages in the art but aredisadvantageous when a study of the distribution of defect levels above the Fermi level is desired. This is so because the information in the image produced by the'photoemissive electron microscope is not produced by electrons from these levels but always by electrons from the different levels below the Fermi level. As a consequence, photoemission type electron microscopes have only limited usages for investigations interested in the surface conditions pertaining to defect levels above the Fermi level of a substance.

SUMMARY OF THE INVENTION The invention overcomes the disadvantages of photoemission electron microscopes by utilizing the phenomena of exoelectron emission from a surface under controlled conditions and then magnifying the image produced by the e'xoelectrons with an electron microscope which is modified to operate in the new environment. In the inventive system, a standard photoemission electron microscope is modified by replacing the intense Ultraviolet Light Source 12 as shown in FIG. 1 with a radiation source such as an electron gun, infrared source, visible light source, an alpha or beta source, or gamma source. Other types of radiation sources can be used if desired. The sample to be investigated is placed in a unit which is capable of lowering the temperature of the sample below room temperature and also which is capable of increasing the temperature of the sample above ambient or room temperature.

After modifying the electron microscope in this manner, the sample is placed in the microscope and cooled to a desired temperature to stabilize inherent electron emission from the sample. The sample is then subjected to the radiation from the available source. This subjection to radiation causes electrons within the surface layer of the sample to rise to levels above the Fermi level. Subsequently, the sample is heated to the temperature at which the particular sample material best emanates exoelectrons and the electrons emanate from the surface of the material. Because the electrons emanate from the surface of the material on a point-to-point basis, and because the surface characteristics of the material are determinative of the number of exoelectrons emanating, the exoelectron distribution across the surface of the sample is characteristic of the surface faults and density inhomogeneities of the surface. The exoelectron pattern having the image information is next subjected to an accelerating field by the use of an anode and a high electric potential so that the electrons are focused and pass through the standard three-stage electron microscope. After passing through the microscope, the electrons are detected to produce a visual or photographic display.

Because exoelectron emanation is used in this type of microscope all electrons come from defect levels above the Fermi level in a thin surface layer of the sample. The contrast of the image is determined by the spatial distribution of these levels along the surface. The contrast of photoemission images is determined by levels below the Fermi level and is therefore different. For this reason the exoelectron microscope gives information about the sample surface that cannot be obtained otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic showing of the prior art photoemission electron microscope.

FIG. 2 is a schematic showing of the modifications made to the prior art electron microscope to make it operate in accordance with the phenomena of exoelectron emission.

DETAILED DESCRIPTION As stated hereinabove under the heading, Background of the Invention, the prior art electron microscope described with respect to FIG. 1 is disadvantageous for usages where information relating to defect levels above the Fermi level in an infinitely thin surface layer of the sample is desired. This is so because the electrons which produce the image formed in photoemission types of electron microscopes emanate from energy levels below the Fermi level. Accordingly, no information relating to defect levels above the Fermi level can be obtained with photoemission electron microscopes. The system shown in FIG. 2 overcomes these disadvantages by replacing the Ultraviolet Source 12 with a different radiation Source 26. Radiation Source 26 can be an electron gun X-ray tube, an alpha, beta or gamma source, an infrared or a visible light source, or any type of source, the output of which will raise electrons within the sample to the level required for exoelectron emission, that is, above the Fermi level.

Sample is enclosed or otherwise supported by a sample Holding Unit 27 so that Sample 10 can be cooled and subsequently heated without changing the positioning of Sample 10 or without physically replacing the cooling unit with a heating unit. The cooling and heating mechanisms can be any of several known in the art, and therefore a detailed description of these devices is not required herein. However, as an example, cooling can be effected with a liquid nitrogen or helium system.

The mode of operation of the exoelectron microscope is a relatively simple multi-step process. Firstly, Sample 10 is cooled to room temperature or any desired temperature below room temperature, which stabilizes the inherent emission of electrons by the sample. This can be done by the use of liquid nitrogen or helium, or any other standard cooling technique. After Sample 10 has been cooled, Radiation Source 26 is actuated so that the entire surface of the sample is illuminated with the desired type of radiation. This radiation can be any of the types mentioned hereinabove with respect to Source 26. The illumination of Sample 10 with the radiation fills the traps along an infinitely thin surface of Sample 10. Radiation Source 26 is then deactuated and Sample 10 heated to the temperature at which exoelectron emission is maximum. This causes the exoelectrons to emanate from the surface of Sample 10 on a point-to-point basis so that the total emitted pattern of electrons carries surface information relating to surface faults or density inhomogeneities, etc., relative to the surface. These electrons are accelerated by the use of high voltage Source 28 which establishes an electric field between the sample and Anode 14, thereby attracting the electrons to Anode 14. Electrons pass through Aperture l6 and are attracted through the electron microscope in the same manner as that described with respect to FIG. 1 until they form an output Image 24.-This image isprojected onto the phosphor screen or a photographic plate to thereby create a visual image of the surface characteristics of Sample 10. These characteristics are highly magnified and thus faults and surface variationswhich are undetectable by the naked eye or with optical microscopes can be detected and viewed.

FIG. 2'shows a microchannel plate interposed between Image 24 and Output Detector 21. The Microchannel Plate 29 is used in order to enhance the intensity of the image and thus improve its viewing characteristics. The microchannel plate is an element which is known in the art. However, briefly described, it contains a large plurality of parallel channels, the interior walls of which are coated with an electron emissive material. When an electron enters the aperture of a channel it causes the emission of secondary electrons so that each electron impinging upon the input surface of the MCP causes the emanation of a large number of output electrons. Because the secondary. electrons emanate from and are propelled through a large number of parallel channels, image information contained within the illuminating radiation is preserved by the microchannel plate, thus resulting in a highly intensified image at the output surfaceof the channel plate.

The inventive microscope can also be used as an induced photoemission electron microscope. For such a usage the sample is located and exposed to ionizing radiation to fill the traps. After filling the traps a source of illumination having a wavelength longer than that required for normal photoemission is used to illuminate the sample. As an example, visible light and infrared light can be used for this purpose. The illumination causes exoelectrons to emanate from the surface of the sample. The pattern of the electrons is representative of the condition of the sample surface, in the same manner as when thermally stimulated.

The inventive exoelectron microscope is capable of revealing surface variations of a sample by the detection of electrons above the Fermi level, while the prior art photocmission microscopes reveal surface variations by the detection of electrons below .the Fermi level. The two types of microscopes thus augment one another. Consequently, maximum knowledge of a sample can be gained by employing both types of microscopes. This is quite simple because a single microscope can be modified to operate on both principles simply by changing the illumination source. Because the inventive exoelectron microscope yields image information by detection of electrons above the Fermi level, it is capable of yielding information heretofore impossible to obtain.

What is claimed is:

1. A surface imaging exoelectron microscope comprising:

means for supporting a sample, at least one surface of which is to be investigated; means for selectively cooling said sample to a temperature below ambient to stabilize electron emission from said surface;

means for illuminating said sample at said below ambient temperature with radiation to cause electrons to be trapped along the surface of said sample above the Fermi level;

means for stimulating said illuminated sample to cause said trapped electrons to be emanated as an exoelectron pattern containing image information representative of surface variation atsaid sample;

means for magnifying said emitted exoelectron pattern;

and means for detecting said magnified exoelectron pattern to produce an output image.

2. The microscope of claim ll wherein said means for stimulating said sample includes means for heating said sample to a temperature above ambient and at which maximum exoelectron emission occurs.

3. The microscope of claim 1 wherein said means for illuminating includes a source of electromagnetic radiation, having sufficient energy to excite. the electrons within the sample material to energy levels above the Fermi level.

4. The microscope of claim 1 wherein said means for illuminating includes a source of energetic particles.

5.-The microscope of claim ll wherein said means for stimulating includes an electromagnetic radiation source having sufficient energy to cause the trapped electrons to emanate from said illuminated sample.

6. The microscope of claim 1 wherein said means for stimulating includes a source of energetic particles.

7. The microscope of claim 1 wherein said means for detecting includes a microchannel plate for intensifying said output image.

8. The microscope of claim 2 wherein said means for detecting includes a microchannel plate for intensifying said output image.

9. The microscope of claim 2 further including radiation means for illuminating said sample to thereby enhance the emanation of exoelectrons from said sample.

l0. The microscope of claim 9 wherein said radiation means is a source of electromagnetic radiation having a frequency range between infrared and visible light, inclusive.

11. The microscope of claim 2 whereinsaid means for illuminating includes a source of electromagnetic radiation. 1

12. The microscope of claim 2 wherein said means for illuminating includes a source of energetic particles.

13. A method of producing images of surface variations of a sample employing an electron microscope to magnify an emitted exoelectron pattern including the steps of:

cooling said sample to a temperature below ambient to stabilize inherent electron emission from said sample;

illuminating said sample with radiation to raise electrons in said sample to an energy level higher than the Fermi level;

heating said sample to a temperature between ambient and the thermal emission temperature of said sample to enhance the escape of said electrons as exoelectrons, said exoelectrons emanating at differing rates along the surface of said sample so that said exoelectrons form a pattern;

magnifying said emitted exoelectron pattern with said microscope to produce said image.

14. The method of claim 13 wherein said illuminating step includes the step of subjecting said sample to electromagnetic energy.

15. The method of claim 13 wherein said illuminating step includes the step of subjecting said sample to energetic particles.

16. The method of claim 13 further including the step of intensifying said emitted exoelectron image by the use of a microchannel plate.

17. The method of claim 15 further including the step of intensifying said emitted exoelectron image by the use of a microchannel plate.

18. A method of producing images of surface variations of a sample employing an electron microscope to magnify an exoelectron emitted pattern including the steps of:

cooling said sample to a temperature below ambient to stabilize inherent electron emission from said sample;

illuminating said sample with radiation to raise electrons in said sample to an energy level higher than the Fermi level;

radiating said sample with energy to enhance the escape of exoelectrons in accordance with surface variations along the surface of said sample so that .said'electrons form a pattern;

magnifying said pattern with said microscope to produce said image.

19. The method of claim 18 wherein said radiating step is performed with electromagnetic energy in the frequency range between infrared and visible light, inelusive.

20. The method of claim m wherein said illuminating step includes the step of illuminating said sample with electromagnetic radiation.

21. The method of claim 11% wherein said illuminating step includes the step of illuminating said sample with energetic particle radiation.

22. The method of claim 20 wherein said radiating step is performed with electromagnetic energy in the portion of the electromagnetic spectrum from infrared to visible light, inclusive.

23. The method of claim 21 wherein said radiating step includes the step of radiating said sample with electromagnetic energy in the portion of the electromagnetic spectrum from infrared to visible light, inclusive.

M. A surface imaging exoelectron microscope comprising:

pattern containing image information representative of the sample surface variations; means for focusing said emitted exoelectron pattern to form a magnified exoelectron pattern; and means for detecting said magnified pattern to produce an output image. 

1. A surface imaging exoelectron microscope comprising: means for supporting a sample, at least one surface of which is to be investigated; means for selectively cooling said sample to a temperature below ambient to stabilize electron emission from said surface; means for illuminating said sample at said below ambient temperature with radiation to cause electrons to be trapped along the surface of said sample above the Fermi level; means for stimulating said illuminated sample to cause said trapped electrons to be emanated as an exoelectron pattern containing image information representative of surface variation at said sample; means for magnifying said emitted exoelectron pattern; and means for detecting said magnified exoelectron pattern to produce an output image.
 2. The microscope of claim 1 wherein said means for stimulating said sample includes means for heating said sample to a temperature above ambient and at which maximum exoelectron emission occurs.
 3. The microscope of claim 1 wherein said means for illuminating includes a source of electromagnetic radiation, having sufficient energy to excite the electrons within the sample material to energy levels above the Fermi level.
 4. The microscope of claim 1 wherein said means for illuminating includes a source of energetic particles.
 5. The microscope of claim 1 wherein said means for stimulating includes an electromagnetic radiation source having sufficient energy to cause the trapped electrons to emanate from said illuminated sample.
 6. The microscope of claim 1 wherein said means for stimulating includes a source of energetic particles.
 7. The microscope of claim 1 wherein said means for detecting includes a microchannel plate for intensifying said output image.
 8. The microscope of claim 2 wherein said means for detecting includes a microchannel plate for intensifying said output image.
 9. The microscope of claim 2 further including radiation means for illuminating said sample to thereby enhance the emanation of exoelectrons from said sample.
 10. The microscope of claim 9 wherein said radiation means is a source of electromagnetic radiation having a frequency range between infrared and visible light, inclusive.
 11. The microscope of claim 2 wherein said means for illuminating includes a source of electromagnetic radiation.
 12. The microscope of claim 2 wherein said means for illuminating includes a source of energetic particles.
 13. A method of producing images of surface variations of a sample employing an electron microscope to magnify an emitted exoelectron pattern including the steps of: cooling said sample to a temperature below ambient to stabilize inherent electron emission from said sample; illuminating said sample with radiation to raise electrons in said sample to an energy level higher than the Fermi level; heating said sample to a temperature between ambient and the thermal emission temperature of said sample to enhance the escape of said electrons as exoelectrons, said exoelectrons emanating at differing rates along the surface of said sample so that said exoelectrons form a pattern; magnifying said emitted exoelectron pattern with said microscope to produce said image.
 14. The method of claim 13 wherein said illuminating step includes the step of subjecting said sample to electromagnetic energy.
 15. The method of claim 13 wherein said illuminating step includes the step of subjecting said sample to energetic particles.
 16. The method of claim 13 further including the step of intensifying said emitted exoelectron image by the use of a microchannel plate.
 17. The method of claim 15 further including the step of intensifying said emitted exoelectron image by the use of a microchannel plate.
 18. A method of producing images of surface variations of a sample employing an electron microscope to magnify an exoelectron emitted pattern including the steps of: cooling said sample to a temperature below ambient to stabilize inherent electron emission from said sample; illuminating said sample with radiation to raise electrons in said sample to an energy level higheR than the Fermi level; radiating said sample with energy to enhance the escape of exoelectrons in accordance with surface variations along the surface of said sample so that said electrons form a pattern; magnifying said pattern with said microscope to produce said image.
 19. The method of claim 18 wherein said radiating step is performed with electromagnetic energy in the frequency range between infrared and visible light, inclusive.
 20. The method of claim 18 wherein said illuminating step includes the step of illuminating said sample with electromagnetic radiation.
 21. The method of claim 18 wherein said illuminating step includes the step of illuminating said sample with energetic particle radiation.
 22. The method of claim 20 wherein said radiating step is performed with electromagnetic energy in the portion of the electromagnetic spectrum from infrared to visible light, inclusive.
 23. The method of claim 21 wherein said radiating step includes the step of radiating said sample with electromagnetic energy in the portion of the electromagnetic spectrum from infrared to visible light, inclusive.
 24. A surface imaging exoelectron microscope comprising: means for supporting a sample, at least one surface of which is to be investigated; means for exciting said sample to trap the electrons along said at least one surface at energy levels above the Fermi level; means for stimulating said surface to cause said trapped electrons to be emitted as an exoelectron pattern containing image information representative of the sample surface variations; means for focusing said emitted exoelectron pattern to form a magnified exoelectron pattern; and means for detecting said magnified pattern to produce an output image. 